Taking the example of an automotive battery as a direct-current power source, automotive batteries ordinarily use a chargeable secondary battery. When the battery and the automotive equipment are expressed by electric circuitry, they are as shown in FIG. 12. In the figure, 20 is a battery, 21 is a self-starting motor, 22 is electrical equipment, 23 is a generator, 24 is a starter switch, 25 is a rectifier diode, and 26 is an automatic on/off switch that is automatically engaged when the electromotive force of the charging generator 23 is equal to or greater than the voltage of the battery.
The battery 20 is expressed by a pure battery Eo and internal resistance r, and a voltage drop e=r i occurs due to an output current i and internal resistance r, such that a voltage V1 of an output terminal becomes V1=Eo−e=Eo−r i
As the loads of the battery 20, there are the self-starting motor 21 and the electrical equipment 22. Inductance components (inductive loads) exist not only in the self-starting motor 21, as a matter of course, but also in the electrical equipment 22 such as a car air conditioner and car audio. Counter electromotive force occurs from these inductance components, and this becomes noise that is superimposed on the voltage of the output terminal.
Furthermore, as the automatic on/off switch 26 provided at the plus side of the charging generator 23 is turned on and off while the voltage of the battery 20 is monitored, a coil L of the charging generator 23 is open during the off times, which impedes absorption of the counter electromotive force that has occurred.
This type of counter electromotive force due to inductance components causes grating noise in car audio and radio speakers, and also causes flickering in car navigation and television images.
Conventionally, as shown, for example, in FIG. 13, noise is reduced by regenerating the noise that occurs in a blower motor 201 to a battery power source VB by means of a diode 206; the harmonic components of the noise are attenuated by a capacitor 224; the noise that occurs between battery power sources due to the switching of a switching element 202 is attenuated by an LC filter configured from an electrolytic capacitor 222 and inductance 223; the magnitude of noise generated when the switching element 202 is off is reduced by means of the inductance 223; and the voltage fluctuations between the battery power sources are mitigated by the electrolytic capacitor 222 (for example, see Japanese Unexamined Patent Application, First Publication No. H07-283797).
However, with noise filters using capacitors and inductances, it is only possible to remove harmonic components and specified frequency components, and there is the problem that there is little inhibiting effect on high-peak counter electromotive force.
Moreover, as conventional noise filters are attached to the electrical equipment itself, there is the problem that they cannot be newly installed in existing electrical equipment.